1. Field of the Invention
This invention relates to curable organopolysiloxane which can be used as a silicone inner coat for protecting the surface of semiconductor elements.
2. Description of Prior Art
By the term "inner coat" of the semiconductor is meant a protecting layer which is pre-coated on the surface of the semiconductor element in order to prevent the damages of the surface of the semiconductor from occurring during the molding of the semiconductor devices with a resin or those caused by the contractile stress generated upon curing of the resin on the semiconductor, or to protect the connection of the bonding wire.
FIG. 1 shows an example of the semiconductor device having the semiconductor element covered with the inner coat.
The semiconductor element 2 having a layer of the inner coat layer 1, which is bonded and fixed with a die bonding adhesive layer 4 onto a lead frame 3a which is used as the semiconductor substrate. Reference numerals 5 and 6 in FIG. 1 denote a bonding wire and a molding material, respectively. The bonding wire 5 is bonded with the semiconductor element 2 and the other lead flames 3b and 3c, and also they are molded in a molding material 6.
Since the surface of the semiconductor element and the surrounding area are processed minutely, they tend to receive various physical stresses such as one generated at the time of assembling transfer mold and the like or one generated as the result of the differences in the coefficient of thermal expansion in the semiconductor due to the change in the temperature in the inside thereof.
Furthermore, the aluminum wiring or the aluminum electrode of the semiconductor element is corroded by contaminants in the molding material, especially Cl ions, which are transported by water that enters into the interface between the molded material and the lead frame through the molding material and the like.
In general, a method is used in which a polyimide resin or a silicone resin is coated as a coating material on the surface of the semiconductor to protect it from the physical stresses and the corrosion, especially the silicone resin is useful for protecting the surface of the semiconductor element and absorbing the physical stresses generated from the surrounding area of the semiconductor element because the silicone resin has particularly low modulus of elasticity.
The protective layer can be formed on the surface of the semiconductor element by curing the silicone resin which is cured after quantitatively dropping it on the surface of the semiconductor element using a dispenser or the like. When the silicone resin is cured on the semiconductor element, the permeation of water from around the semiconductor element onto the surface thereof cannot be avoided because of the low tackiness between the silicone resin and the surface of the semiconductor element, gold wire and molding material. Thus, the aluminum wire or the aluminum electrode on the surface of the semiconductor element tend to be corroded by the impurities, especially Cl ions transported by water, which results in that the reliability of the semiconductor is reduced.
Accordingly, it has heretofore been proposed to improve the reliability of the semiconductor in general by the addition of an additive for improving adhesion strength which is so-called a tackifier into the silicone resin in order to improve the adhesion strength of the silicone resin with the semiconductor, gold wire, and molding material.
In general, the conventionally used tackifier, which includes silane-coupling agent, siloxane oligomer and the like, has a low molecular weight and a relatively low boiling point, so that it is easily volatilized and scattered over the surface of the semiconductor when the resin is heated for curing, thus contaminating the lead frame and the like.
When the lead frame is molded with a resin-type material such as an epoxide resin and the like, the adhesion strength between the lead frame and the molding material decreases once contamination of the lead frame occurs, causing permeation of water into the semiconductor element. For instance, when an examination is conducted under the limited temperature and humidity conditions, such as pressure cooker test under 121.degree. C./2 atm abs., the reliability of the semiconductor element decreases due to remarkable permeation of the contaminated water from the interface between the lead frame and the molding material.